Transmission electron microscopy of a geopolymer phase, derived from metakaolin and alkaline silicate solutions and having nominal Na/Al and Si/Al molar ratios of 1 and 2, respectively, showed it to be amorphous on the similar to 1 nm scale after curing at 40 degrees C. In samples containing 5 wt% Cs or Sr, Cs inhabited the amorphous phase, whereas Sr was incorporated only partly, being preferentially partitioned to crystalline SrCO3. Heating progressively to dewater the material had little effect on the overall structure for temperatures up to 500 degrees C. From solid-state nuclear magnetic resonance in material cured at near-ambient temperatures, Cs, like Na, was found to be basically associated with pore water, but with significant linkage to the aluminosilicate framework, more so than for Na. Subsequent heating to 300 degrees C increased the linkage of Cs and Na to the framework.